Semiconductor assembly with conductive rim and method of producing the same

ABSTRACT

An apparatus has first and second wafers, and a conductive rim between the first and second wafers. The conductive rim electrically and mechanically connects the first and second wafers. In addition, the conductive rim and second wafer at least in part seal an area on the surface of the first wafer.

PRIORITY

This patent application claims priority from and is a divisional patentapplication of U.S. patent application Ser. No. 10/737,231, filed Dec.15, 2003, entitled, “SEMICONDUCTOR ASSEMBLY WITH CONDUCTIVE RIM ANDMETHOD OF PRODUCING THE SAME,” and naming Susan A. Alie, Bruce K.Wachtmann, Michael Judy, and David Kneedler as inventors, the disclosureof which is incorporated herein, in its entirety, by reference.

FIELD OF THE INVENTION

The invention generally relates to electronic devices and, moreparticularly, the invention relates to capped electronic devices.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (i.e., “MEMS”) are highly miniaturizeddevices that can be configured to perform a wide variety of functions.For example, a MEMS device can be implemented as an accelerometer tomeasure the acceleration of a moving body. One type of MEMSaccelerometer, for example, has a suspended mass that, in response to anacceleration, moves relative to an underlying substrate. Accelerationthus may be calculated as a function of the movement of the suspendedmass relative to its underlying substrate.

Because of their relatively small size, the mechanical structures ofMEMS devices (e.g., the suspended mass in the above noted accelerometer)typically are both fragile and sensitive. Accordingly, many seeminglyinnocuous things can adversely impact MEMS performance, such as dust,moisture, and static electricity. The art has responded to this problemby isolating sensitive MEMS mechanical structures from the outsideenvironment. For example, one isolation method bonds a cap over themechanical components.

Although generally satisfactory for isolating MEMS structure, thissolution creates other problems. In particular, the cap can accumulate arelatively large static electric charge that can adversely affect deviceperformance and/or damage the MEMS device.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, an apparatus has firstand second wafers, and a conductive rim between the first and secondwafers. The conductive rim electrically and mechanically connects thefirst and second wafers. In addition, the conductive rim and secondwafer at least in part seal an area on the surface of the first wafer.

In some embodiments, the conductive rim and second wafer hermeticallyseal the area on the surface of the first wafer. Among other things, theconductive rim can be a silicide.

The sealed area on the first wafer may include a number of things, suchas MEMS structure. In that case, the conductive rim illustratively iselectrically isolated from the MEMS structure. Moreover, the secondwafer may include a cap, which may in part protect the MEMS structurefrom the environment.

In some embodiments, the first wafer includes circuitry capable ofdelivering a bias voltage to the second wafer via the conductive rim.The circuitry may have a maximum temperature to which it can be exposed.In such case, the conductive rim illustratively is produced from amaterial that interdiffuses and/or melts at a temperature that is lessthan the maximum temperature to which the circuitry can be exposed.

At least one of the first wafer and the second wafer may be producedfrom a silicon based material. For example, at least one of the firstwafer and the second wafer may be comprised of polysilicon, singlecrystal silicon, or silicon germanium.

In accordance with another aspect of the invention, a method of forminga MEMS device places rim material between a first wafer and a secondwafer to form an intermediate apparatus. The rim material is placed sothat it forms a closed loop defining an area on the first wafer. Themethod then both heats and applies pressure to the intermediateapparatus. After heating and applying pressure, the rim materialcooperates with the first and second wafers to both seal the area on thefirst wafer and electrically connect the first and second wafers.

In illustrative embodiments, the second wafer is substantially parallelwith the first wafer. The rim material may cooperate with the first andsecond wafers by integrating with the first and second wafers to form acomposite material that comprises the rim material. For example, the rimmaterial may diffuse into the first and second wafers to form a silicidematerial.

The method may form MEMS structure within the area on the first wafer.Circuitry also may be formed on the first wafer. Among other things, thecircuitry may be capable of applying a bias voltage to the second waferthrough the rim material.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and advantages of the invention will be appreciated morefully from the following further description thereof with reference tothe accompanying drawings wherein:

FIG. 1 schematically shows a perspective view of an electronic devicethat may be capped in accordance with illustrative embodiments of theinvention.

FIG. 2 schematically shows a cross-sectional view of the device shown inFIG. 1 along line X-X, where the device is configured in accordance withone embodiment of the invention.

FIG. 3 shows a process of forming the electronic device shown in FIG. 2in accordance with various embodiments of the invention.

FIG. 4 schematically shows a cross-sectional view of an alternativeembodiment of the invention implemented on MEMS device having circuitryand structure on the same die.

FIG. 5 schematically shows a cross-sectional view of another alternativeembodiment of the invention implemented on MEMS device having circuitryand structure on the same die and circuitry on the cap.

FIG. 6 schematically shows a cross-sectional view of yet anotheralternative embodiment of the invention implemented on a MEMS device inwhich the cap effectively forms another MEMS device.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In illustrative embodiments, a MEMS device has a conductive rim thatsecures a cap to a MEMS die. The conductive rim both seals MEMSstructure on the MEMS die and electrically connects the cap with groundor a fixed bias potential. Among other things, the conductive rim may beformed at least in part from a silicide that integrally couples the capand MEMS die. Details of various embodiments are discussed below.

FIG. 1 schematically shows a generic MEMS device 10 that may beconfigured in accordance with illustrative embodiments of the invention.The MEMS device 10 shown includes a capped MEMS die 12 coupled with acircuit die 14. Accordingly, the MEMS die 12 shown has structure that iscontrolled and/or monitored by circuitry on the circuit die 14. Thecircuit die 14 has bond pads 16 or other interconnects to electricallycommunicate with an external device, such as a computer. To furtherprotect the MEMS device 10 from the environment, conventional processesmay mount the entire MEMS device 10 within a package.

The MEMS device 10 may be any conventionally known MEMS device 10, suchas an inertial sensor. For example, the MEMS device 10 may be agyroscope or an accelerometer. Exemplary MEMS gyroscopes are discussedin greater detail in U.S. Pat. No. 6,505,511, which is assigned toAnalog Devices, Inc. of Norwood, Mass. Exemplary MEMS accelerometers arediscussed in greater detail in U.S. Pat. No. 5,939,633, which also isassigned to Analog Devices, Inc. of Norwood, Mass. The disclosures ofU.S. Pat. Nos. 5,939,633 and 6,505,511 are incorporated herein, in theirentireties, by reference.

FIG. 2 schematically shows a cross-sectional view of the device shown inFIG. 1 along line X-X. Specifically, the MEMS device 10 includes theabove noted MEMS die 12 (e.g., comprised of a silicon-based material,such as silicon) having a silicon-based cap 18 mounted to its top side,and the above noted circuit die 14 mounted to its bottom side. The cap18 illustratively is formed from polysilicon and etched to have a cavity20 defined by a cap rim 22 extending from its interior side. The cavity20 overlies a central portion of the MEMS die 12, which includes theprimary structure 24 for effectuating the MEMS functionality. Forexample, if the MEMS die 12 is an accelerometer, the structure 24 mayinclude a movable mass suspended above a substrate 26.

In accordance with illustrative embodiments, the MEMS device 10 isconsidered to form a conductive rim 28 that circumscribes the MEMSstructure 24 on the MEMS die 12. Among other things, the conductive rim28 forms a hermetic seal that protects the MEMS structure 24 from theenvironment. For example, the hermetic seal may protect the structure 24from dust, moisture, and dirt. In alternative embodiments, theconductive rim 28 provides a non-hermetic seal to the MEMS structure 24.As known by those in the art, a non-hermetic seal may protect the MEMSstructure 24 from dust and dirt, but it is not moisture impervious.

In addition to sealing the MEMS structure 24, the conductive rim 28 alsoelectrically connects the cap 18 with the circuit die 14 through theMEMS die 12. The MEMS die 12 thus includes vias 32 and contacts 34(e.g., balls of a ball grid array) that extend from the conductive rim28, through the MEMS die 12, and to the circuit die 14. Circuitry 30 onthe circuit die 14 sets the potential of the cap 18 to ground or anydesired voltage level.

To those ends, the MEMS die 12 has a rim of polysilicon material(extending from the substrate 26 and referred to herein as the “MEMS rim36”) that integrally couples with the cap rim 22 extending from the cap18. In illustrative embodiments, the cap rim 22 and MEMS rim 36 meet ata loosely defined intersection region 38 having a relatively highsilicide concentration (discussed below with regard to FIG. 3). Theconcentration of silicide may be highest at the center of thatintersection region 38 and reduce to essentially zero at its looselydefined ends. Moreover, the conductive rim 28 (formed from theintersection region 38, MEMS rim 36 and cap rim 22) preferably iselectrically isolated from the MEMS structure 24 to ensure that thepotential applied to the cap 18 is carefully controlled.

In illustrative embodiments, the MEMS rim 36 also acts as a sensorelement. Alternatively, among other things, the MEMS rim 36 may act as aground plane element, a circuit element, or dummy mechanical structure.

FIG. 3 schematically shows an illustrative process of forming the MEMSdevice 10 shown in FIG. 2. It should be noted that various steps of thisprocess may be performed in a different order than that discussed. Inaddition, those skilled in the art should understand that additionalsteps may be performed, while others may be omitted.

The process begins at step 300, in which the MEMS die 12 is formed inaccordance with conventional processes. Among other processes,conventional surface micromachining processes may be used to form theMEMS die 12. Alternatively, silicon-on-insulator (“SOI”) processes maybe used. As noted above, the die may be formed from a silicon-basedmaterial, such as polysilicon. In alternative embodiments, however,other types of materials may be used. For example, single crystalsilicon or silicon germanium may be used for all or selected portions ofthe MEMS die 12. In any case, the conductivity of the MEMS rim 36 shouldbe controlled to be a satisfactory level. If necessary, some doping maybe required to ensure appropriate conductive properties.

The cap 18 then is formed at step 302. In a manner similar to the MEMSdie 12, the cap 18 may be formed from a polysilicon wafer (or othermaterial) in accordance with conventional processes (e.g., surfacemicromachining processes). The sizes of the cavity 20 and cap rim 22illustratively are selected to ensure a sufficient clearance with theMEMS die 12. In particular, the cavity 20 should be sufficiently largeenough to not interfere with movement of the structure 24 on the MEMSdie 12.

The process then continues to step 304, in which conventional processesform the circuit die 14. Any conventional circuitry designed to performthe desired function can be used. For example, the circuitry shown inthe above noted incorporated patents can be used. In particular, if theMEMS device 10 is an accelerometer, then the circuitry shown in U.S.Pat. No. 5,939,633 can be used to sense electrostatic changes in theMEMS die 12. It should be noted that in illustrative embodiments, theMEMS die 12, cap 18 and circuit die 14 each are formed as one of anarray of identical elements on a single wafer.

Conventional processes then may metalize the bottom side of the cap 18(step 306). For example, a layer of platinum 40 may be sputter depositedonto the bottom side of the cap 18. The metalized cap 18 then may beplaced on the MEMS die 12 so that the MEMS rim 36 directly contacts thecap rim 22.

At this point in the process, the MEMS die 12 and cap 18 are not securedtogether. Accordingly, to fuse them together, this intermediateapparatus is subjected to relatively high temperatures and pressures (atstep 308) sufficient to form a silicide bond in the intersection region38. Those skilled in the art should be able to select the appropriatetemperatures and pressures. By way of example only, subjecting theintermediate apparatus to temperatures of between about 280-450 degreesC. and pressures of about two atmospheres for about forty to fiftyminutes should provide satisfactory results.

This step in the process thus produces platinum-silicide in theintersection region 38. As known by those in the art, because of theinter-diffusion of the platinum into the polysilicon, the outer portionsof the two rims and the platinum between the two rims cooperate to forma substantially integral and conductive connector. The concentration ofplatinum thus is highest in the center of the intersection region 38(e.g., fifty percent platinum and fifty percent polysilicon), while itreduces to zero as a function of distance from the center.

Materials other than platinum may be used to produce the silicide bond.For example, tungsten or titanium may be used. Use of such notedmaterials, however, typically requires higher temperatures to form theirrespective silicide bonds than those required of platinum. Accordingly,use of tungsten or titanium with embodiments that have circuitry 30 onthe intermediate apparatus (e.g., see FIGS. 4-6, discussed below) maynot be recommended because such high temperatures may adversely affectthe circuitry 30. In other words, the material selected to form thesilicide bond should interdiffuse (and/or melt) at a temperature that islower than temperatures that can adversely impact the circuitry 30 orother temperature sensitive portions of the MEMS device 10.

Other types of bonds can be used. For example, rather than form asilicide bond, a solder-based bond can be used. Use of this type ofbond, however, requires additional process steps. In particular, inaddition to metalizing at least the cap rim 22 (as discussed above), theMEMS rim 36 also is metalized. Continuing with the above example, in amanner similar to the cap rim 22, the MEMS rim 36 also may be sputterdeposited with platinum or other solderable material. Solder then can beapplied and cured at relatively low temperatures.

As noted above, the conductive rim 28 illustratively completelyencircles the MEMS structure 24 to provide both a hermetic seal and aconductive path between the cap 18 and circuitry 30. In someembodiments, the conductive rim 28 forms a circular ring around thestructure 24. In other embodiments, the conductive rim 28 forms someother shape (e.g., oval, rectangular, or an irregular shape) around theMEMS structure 24.

After the silicide bond is formed, the process continues to step 310, inwhich the bottom portion of the MEMS die 12 (or wafer, as the case maybe) is subjected to a thinning process (e.g., backgrinding or etch backprocesses) that exposes the vias 32. Conductive contacts 34 then can bemounted to the bottom of the vias 32 (step 312), which then can bemounted to corresponding contacts on the top surface of the circuit die14 (step 314). The wafers then can be diced, thus completing theprocess. As noted above, after it is fully formed, the resulting MEMSdevice 10 may be mounted in a package, flip chip mounted on a circuitboard (after contacts are formed on one side), or used in anyconventional manner.

Accordingly, as shown in FIG. 2 and discussed above, the cap 18 iselectrically connected to the circuit die 14 through the conductive rim28, corresponding vias 32, and ball contact 34 on the bottom of the MEMSsubstrate 26. Circuitry 30 on the circuit die 14 can set the potentialof the cap 18 to ground or any desired voltage level. Alternatively, theconductive path through the MEMS die 12 to the conductive rim 28 canterminate at an external pin that can be externally grounded or set toany desired potential, such as 0.2 volts.

FIG. 4 shows an alternative embodiment of the MEMS device 10. Ratherthan have a separate circuit die 14, the MEMS die 12 may include bothstructure 24 and circuitry 30. In a manner similar to the MEMS die 12shown in FIG. 2, the MEMS die 12 of this embodiment may be produced in aconventional manner, such as by using surface micromachining or SOIprocesses.

FIG. 5 shows another embodiment in which a “smart cap” is used. Inparticular, the cap 18 may have circuitry 30 that shares or complimentsprocessing with the circuitry 30 in the MEMS die 12. Instead of, or inaddition to, bias potentials, the conductive path between the cap 18 andMEMS die 12 also can transmit data messages. FIG. 6 shows yet anotherembodiment in which two MEMS die 12 are mounted together. It should benoted that components of various of the embodiments shown in FIGS. 1-6can be combined to form other embodiments. For example, instead of MEMSdie 12 with integrated circuitry 30, the circuitry 30 in FIG. 6 may belocated on one or more separate circuit die 14 as in FIG. 2.

Generally speaking, various embodiments of the invention have aconductive rim 28 that couples two wafers together in a manner thatseals an internal area (e.g., having sensitive structure 24) on at leastone of the wafers. In addition to (or instead of) other conductivepaths, that conductive rim 28 also can be used to electricallycommunicate at least one of the wafers with some other wafer.Accordingly, unintended potential differences between the cap 18 andMEMS structure 24 should be avoided.

Different materials than those discussed above also may be used.Moreover, some embodiments are applicable to devices other than MEMSdevices. For example, integrated circuits and other types of devices mayimplement aspects of the invention. Accordingly, discussion of MEMSdevices is exemplary and thus, not intended to limit all embodiments ofthe invention.

Although the above discussion discloses various exemplary embodiments ofthe invention, it should be apparent that those skilled in the art canmake various modifications that will achieve some of the advantages ofthe invention without departing from the true scope of the invention.

1. A method of forming a MEMS device, the method comprising: placing rimmaterial between a first wafer and a second wafer to form anintermediate apparatus, the rim material forming a closed loop definingan area on the first wafer; applying pressure to the intermediateapparatus; and heating the intermediate apparatus, after heating andapplying pressure, the rim material cooperating with the first andsecond wafers to seal the area on the first wafer and electricallyconnect the first and second wafers.
 2. The method as defined by claim 1wherein the second wafer is substantially parallel with the first wafer.3. The method as defined by claim 1 wherein the rim material cooperateswith the first and second wafers by integrating with the first andsecond wafers to form a composite material comprising the rim material.4. The method as defined by claim 1 wherein the rim material diffusesinto the first and second wafers to form a silicide material.
 5. Themethod as defined by claim 1 further comprising: forming MEMS structurewithin the area on the first wafer.
 6. The method as defined by claim 1further comprising forming circuitry on the first wafer capable ofapplying a bias voltage to the second wafer through the rim material. 7.A MEMS device formed by the method defined by claim
 1. 8. The method asdefined by claim 1 wherein rim material cooperates with the first andsecond wafers to hermetically seal the area on the first wafer.
 9. Themethod as defined by claim 1 wherein the closed loop forms an irregularshape.